B lymphocytes in lymph nodes and peripheral blood are important for binding immune complexes containing HIV-1

Complement and the control of HIV infection: an evolving story

PURPOSE OF REVIEW

Thirty years ago, investigators isolated and later determined the structure of HIV-1 and its envelope proteins. Using techniques that were effective with other viruses, they prepared vaccines designed to generate antibody or T-cell responses, but they were ineffective in clinical trials. In this article, we consider the role of complement in host defense against enveloped viruses, the role it might play in the antibody response and why complement has not controlled HIV-1 infection.

RECENT FINDINGS

Complement consists of a large group of cell-bound and plasma proteins that are an integral part of the innate immune system. They provide a first line of defense against microbes and also play a role in the immune response. Here we review the studies of complement-mediated HIV destruction and the role of complement in the HIV antibody response.

SUMMARY

HIV-1 has evolved a complex defense to prevent complement-mediated killing reviewed here. As part of these studies, we have discovered that HIV-1 envelope, on administration into animals, is rapidly broken down into small peptides that may prove to be very inefficient at provident the type of antigenic stimulation that leads to an effective immune response. Improving complement binding and stabilizing envelope may improve the vaccine response.

HIV-1 Trans Infection of CD4(+) T Cells by Professional Antigen Presenting Cells

Since the 1990s we have known of the fascinating ability of a complex set of professional antigen presenting cells (APCs; dendritic cells, monocytes/macrophages, and B lymphocytes) to mediate HIV-1 trans infection of CD4(+) T cells. This results in a burst of virus replication in the T cells that is much greater than that resulting from direct, cis infection of either APC or T cells, or trans infection between T cells. Such APC-to-T cell trans infection first involves a complex set of virus subtype, attachment, entry, and replication patterns that have many similarities among APC, as well as distinct differences related to virus receptors, intracellular trafficking, and productive and nonproductive replication pathways. The end result is that HIV-1 can sequester within the APC for several days and be transmitted via membrane extensions intracellularly and extracellularly to T cells across the virologic synapse. Virus replication requires activated T cells that can develop concurrently with the events of virus transmission. Further research is essential to fill the many gaps in our understanding of these trans infection processes and their role in natural HIV-1 infection.

The good and evil of complement activation in HIV-1 infection

The complement system, a key component of innate immunity, is a first-line defender against foreign pathogens such as HIV-1. The role of the complement system in HIV-1 pathogenesis appears to be multifaceted. Although the complement system plays critical roles in clearing and neutralizing HIV-1 virions, it also represents a critical factor for the spread and maintenance of the virus in the infected host. In addition, complement regulators such as human CD59 present in the envelope of HIV-1 prevent complement-mediated lysis of HIV-1. Some novel approaches are proposed to combat HIV-1 infection through the enhancement of antibody-dependent complement activity against HIV-1. In this paper, we will review these diverse roles of complement in HIV-1 infection.

Complement and its role in protection and pathogenesis of flavivirus infections

The complement system is a family of serum and cell surface proteins that recognize pathogen-associated molecular patterns, altered-self ligands, and immune complexes. Activation of the complement cascade triggers several antiviral functions including pathogen opsonization and/or lysis, and priming of adaptive immune responses. In this review, we will examine the role of complement activation in protection and/or pathogenesis against infection by Flaviviruses, with an emphasis on experiments with West Nile and Dengue viruses.

Complement and antibodies: a dangerous liaison in HIV infection?

Due to ongoing recombination and mutations, HIV permanently escapes from neutralizing antibody (nAb) responses of the host. By the masking of epitopes or shedding of gp120, HIV-1 further impedes an efficient neutralization by Abs. Therefore, nAbs responses of the host are chasing behind a rapidly evolving virus and mainly non-neutralizing antibodies (non-nAbs) are present in the host. At the same time, complement deposition on immune-complexed HIV may counteract the immune response by enhancing the infection. On the other hand, complement-mediated lysis is a putative effector mechanism to control viral replication. Here we review the complex interplay between complement, neutralizing and non-neutralizing Abs during HIV infection and discuss the contribution of Abs and complement in blocking versus enhancing the course of infection.

Complement-HIV interactions during all steps of viral pathogenesis

Upon crossing the endothelial barrier of the host, HIV initiates immediate responses of the immunity system. Among its components, the complement system is one of the first the first elements, which are activated to affect HIV propagation. Complement participates not only in the early phase of the immune response, but its effects can be observed continuously and also concern the induction and modification of the adaptive immune response. Here we discuss the role of complement in early and late stages of HIV pathogenesis and review the escape mechanisms, which protect HIV from destruction by the complement system.

Factor I-mediated processing of complement fragments on HIV immune complexes targets HIV to CR2-expressing B cells and facilitates B cell-mediated transmission of opsonized HIV to T cells

Our study demonstrates that binding of complement-opsonized HIV to complement receptor type 1 on human erythrocytes (E) via C3b fragments is followed by a rapid normal human serum-mediated detachment of HIV from E. The release was dependent on the presence of factor I indicating a conversion of C3b fragments to iC3b and C3d on the viral surface. This in turn resulted in an efficient binding of opsonized HIV to CR2-expressing B cells, thus facilitating B cell-mediated transmission of HIV to T cells. These data provide a new dynamic view of complement opsonization of HIV, suggesting that association of virus with E might be a transient phenomenon and the factor I-mediated processing of C3b to iC3b and C3d on HIV targets the virus to complement receptor type 2-expressing cells. Thus, factor I in concert with CR1 on E and factor H in serum due to their cofactor activity are likely to be important contributors for the generation of C3d-opsonized infectious HIV reservoirs on follicular dendritic cells and/or B cells in HIV-infected individuals.

Simian immunodeficiency virus (SIV)/immunoglobulin G immune complexes in SIV-infected macaques block detection of CD16 but not cytolytic activity of natural killer cells

Natural killer cells are components of the innate immune system that play an important role in eliminating viruses and malignant cells. Using simian immunodeficiency virus (SIV) infection of macaques as a model, flow cytometry revealed a gradual loss of CD16+ NK cell numbers that was associated with disease progression. Of note, the apparent loss of NK cells was detected in whole-blood samples but not in isolated peripheral blood mononuclear cells (PBMC), suggesting that an inhibitor(s) of the antibody used to detect CD16, the low-affinity immunoglobulin G (IgG) receptor, was present in blood but was removed during PBMC isolation. (Actual decreases in CD16+ cell numbers in PBMC generally were not detected until animals became lymphopenic.) The putative decrease in CD16+ cell numbers in whole blood correlated with increasing SIV-specific antibody titers and levels of plasma virion RNA. With the addition of increasing amounts of plasma from progressor, but not nonprogressor, macaques to PBMC from an uninfected animal, the apparent percentage of CD16+ cells and the mean fluorescence intensity of antibodies binding to CD16 declined proportionately. A similar decrease was observed with the addition of monomeric IgG (mIgG) and IgG immune complexes (IgG-ICs) purified from the inhibitory plasma samples; some of the ICs contained SIV p27(gag) antigen and/or virions. Of interest, addition of purified IgG/IgG-ICs to NK cell lytic assays did not inhibit killing of K562 cells. These results indicate that during progressive SIV and, by inference, human immunodeficiency virus disease, CD16+ NK cell numbers can be underestimated, or the cells not detected at all, when one is using a whole-blood fluorescence-activated cell sorter assay and a fluorochrome-labeled antibody that can be blocked by mIgG or IgG-ICs. Although this blocking had no apparent effect on NK cell activity in vitro, the in vivo effects are unknown.

DC-SIGN on B lymphocytes is required for transmission of HIV-1 to T lymphocytes

Infection of T cells by HIV-1 can occur through binding of virus to dendritic cell (DC)-specific ICAM-3 grabbing nonintegrin (DC-SIGN) on dendritic cells and transfer of virus to CD4⁺ T cells. Here we show that a subset of B cells in the blood and tonsils of normal donors expressed DC-SIGN, and that this increased after stimulation in vitro with interleukin 4 and CD40 ligand, with enhanced expression of activation and co-stimulatory molecules CD23, CD58, CD80, and CD86, and CD22. The activated B cells captured and internalized X4 and R5 tropic strains of HIV-1, and mediated trans infection of T cells. Pretreatment of the B cells with anti–DC-SIGN monoclonal antibody blocked trans infection of T cells by both strains of HIV-1. These results indicate that DC-SIGN serves as a portal on B cells for HIV-1 infection of T cells in trans. Transmission of HIV-1 from B cells to T cells through this DC-SIGN pathway could be important in the pathogenesis of HIV-1 infection.

A cell surface molecule, DC-SIGN, is known to bind the AIDS virus, human immunodeficiency virus 1 (HIV-1), on dendritic cells. HIV-1 can then be transferred from these dendritic cells to CD4⁺ T cells, in which the virus replicates and kills the T cells. Here, Rappocciolo and colleagues present their findings that DC-SIGN serves a similar function on a subset of B cells of the peripheral blood and tonsils. Although B cells that express DC-SIGN do not replicate HIV-1, they serve as portals for transfer and enhanced HIV-1 infection of CD4⁺ T cells, the major site of virus replication in the host. This newly described pathway for HIV-1 infection of T cells via B cells could be important in the pathogenesis of the virus infection.

Complement-opsonized HIV: the free rider on its way to infection

The complement system (C) is one of the main humoral components of innate immunity. Three major tasks of C against invading pathogens are: (i) lysis of pathogens by the formation of the membrane attack complex (MAC); (ii) opsonization of pathogens with complement fragments to favor phagocytosis; and (iii) attraction of inflammatory cells by chemotaxis. Like other particles, HIV activates C and becomes opsonized. To escape complement-mediated lysis, HIV has adopted various properties, which include the acquisition of HIV-associated molecules (HAMs) belonging to the family of complement regulators, such as CD46, CD55, CD59, and the interaction with humoral regulatory factors like factor H (fH). Opsonized virus may bind to complement receptor positive cells to infect them more efficiently or to remain bound on the surface of such cells. In the latter case HIV can be transmitted to cells susceptible for infection. This review discusses several aspects of C-HIV interactions and provides a model for the dynamics of this process.

HIV and human complement: inefficient virolysis and effective adherence

Both, HIV envelope proteins gp120 and gp41 can directly activate complement system, even in the absence of HIV-specific antibodies. During the budding process HIV acquires host membrane-associated molecules among these complement regulatory proteins (CRPs). The presence of CRPs on the viral surface rescues HIV from complement-mediated virolysis. The inefficient virolysis results in the deposition of complement-fragments on the viral surface allowing interactions of HIV with complement receptor expressing cells. In this review, the interaction of HIV with the complement system and the consequences of complement opsonisation on virus infection will be discussed.

Role of complement in the control of HIV dynamics and pathogenesis

In all ex vivo preparations of HIV tested so far, C3 fragments and, after seroconversion, antibodies were detected on the viral surface. This indicates that HIV survives complement-mediated lysis. The virus has adopted different protection mechanisms to keep complement activation under the threshold necessary to induce virolysis. Among them are complement regulatory proteins that remain functionally active on the surface of HIV and turn down the complement cascade and serum proteins with complement regulatory activities. Therefore, opsonized virions accumulate in HIV-infected individuals, and subsequently adhere to complement receptor (CR) expressing cells. Among them are B cells, which bind opsonized virus. Such bound virus is efficiently transferred to autologous T cells, which subsequently are infected. Other cells interacting via CR with opsonized HIV are follicular dendritic cells (FDC). As shown by ex vivo experiments, up to 80% of virus is bound to follicular dendritic cells through C3-CR interactions. In the brain, HIV is not only interacting with complement proteins, but is able to induce their expression. Thus, interaction of HIV with the complement system is a main mechanism for pathogenesis to AIDS, since retention of (complement-resistant) opsonized viral particles on cell surfaces via CRs occurs in different compartments in HIV-infected individuals, thereby promoting transmission of virus to other permissive cells.

Retention of antigen on follicular dendritic cells and B lymphocytes through complement-mediated multivalent ligand-receptor interactions: theory and application to HIV treatment

In HIV-infected patients, large quantities of HIV are associated with follicular dendritic cells (FDCs) in lymphoid tissue. During antiretroviral therapy, most of this virus disappears after six months of treatment, suggesting that FDC-associated virus has little influence on the eventual outcome of long-term therapy. However, a recent theoretical study using a stochastic model for the interaction of HIV with FDCs indicated that some virus may be retained on FDCs for years, where it can potentially reignite infection if treatment is interrupted. In that study, an approximate expression was used to estimate the time an individual virion remains on FDCs during therapy. Here, we determine the conditions under which this approximation is valid, and we develop expressions for the time a virion spends in any bound state and for the effect of rebinding on retention. We find that rebinding, which is influenced by diffusion, may play a major role in retention of HIV on FDCs. We also consider the possibility that HIV is retained on B cells during therapy, which like FDCs also interact with HIV. We find that virus associated with B cells is unlikely to persist during therapy.

Cellular factors influence the binding of HIV type 1 to cells

The goal of this study was to determine the importance of cellular factors for binding of HIV to cells. HIV primary isolates (PIs) produced in peripheral blood mononuclear cells (PBMCs) bound at relatively high levels to PBMCs but at low levels to cell lines, whereas T cell line-adapted (TCLA) virus produced in the H9 T cell line bound at high levels to both cell lines and PBMCs. Expression of CD4 in CD4-negative cells or blocking CD4 with antibody on CD4-positive cells did not affect virus binding. Blocking of gp120/gp41 with antibodies or a lack of expression of gp120/gp41 in virus particles also did not affect virus binding. However, the cell type from which virus was produced did affect virus binding. Thus, the binding pattern of TCLA virus shifted to that of a PI virus when produced in PBMCs. A PI binding pattern also occurred when a cloned TCLA virus (NL4-3) was produced in PBMCs, indicating that the virus-producing cell type has more of an effect on virus binding than the virus strain. These experiments show that both the virus-producing cell and the target cell have a major influence on HIV binding and suggest that host cell factors incorporated into virions are important for virus binding.

DC-SIGN interactions with human immunodeficiency virus: virus binding and transfer are dissociable functions

The C-type lectins DC-SIGN and DC-SIGNR capture and transfer human immunodeficiency virus (HIV) to susceptible cells, although the underlying mechanism is unclear. Here we show that DC-SIGN/DC-SIGNR-mediated HIV transmission involves dissociable binding and transfer steps, indicating that efficient virus transmission is not simply due to tethering of virus to the cell surface.

Association of immune complexes and plasma viral load with CD4+ cell depletion, CD8+ DR+ and CD16+ cell counts in HIV+ hemophilia patients. Implications for the immunopathogenesis of HIV-induced CD4+ lymphocyte depletion

OBJECTIVE

There is evidence that HIV induces CD4+ depletion in part by the formation of immune complexes (IC) that attach to CD4+ blood lymphocytes. In the present study we examined the relationship of IC-coated CD4+ blood cells with retroviral replication in HAART-treated patients.

PATIENTS AND METHODS

52 hemophilia patients were studied from 1997 to 1999. Lymphocyte subsets, IgM, IgG and gp120 on CD4+ blood cells, in vitro responses of lymphocytes to mitogens, plasma neopterin and plasma viral load were measured.

RESULTS

Patients with detectable viral replication and without ICs on CD4+ blood lymphocytes had a lower viral load (4100 versus 21000 HIV-1 mRNA copies/ml; P = 0.079) and higher CD4+ cell counts (310/microl versus 161/microl; P = 0.035) than patients with ICs on circulating CD4+ lymphocytes. Among patients with < 80 HIV-1 mRNA copies/ml, IC- individuals had slightly higher CD4+ lymphocyte counts than IC+ patients (384/microl versus 316/microl; n.s.). Further evidence for the clinical relevance of the ICs was obtained when 18 patients who had an undetectable viral load at previous investigations were analyzed. Among patients with a stable undetectable viral load, CD4+ counts increased in 6 of 8 IC- but in none of 2 IC+ individuals. In patients whose viral load increased during the observation period, 5 of 6 IC- but none of 2 IC+ individuals showed higher CD4+ cell counts. Impaired virus killing is suggested by lower CD16+ (35/microl versus 107/microl; P = 0.016), higher CD3+ DR+ (178/microl versus 66/microl; P = 0.006), and higher CD8+ DR+ (142/microl versus 34/microl; P = 0.017) cell counts in IC(-) patients compared to IC- patients without detectable viral load. Strong retroviral replication induced strong T cell dysfunctions. Fewer CD3+ 25+ blood lymphocytes (19/microl versus 47/microl; P = 0.006) and a lower in vitro response of T lymphocytes to the mitogens Con A (RR: 0.3 versus 1.2; P=0.023) and CD3 mab (RR: 0.5 versus 2.4; P = 0.012) was observed in IC+ patients with detectable versus undetectable viral load.

CONCLUSION

Our data suggest that ICs on circulating CD4+ blood lymphocytes are primarily associated with CD4+ lymphocyte depletion whereas the plasma viral load is primarily associated with decreased T lymphocyte activation, lower CD16+ counts, and higher CD8+ DR+ lymphocytes which might be the effector cells for virus elimination.

B cell-mediated infection of stimulated and unstimulated autologous T lymphocytes with HIV-1: role of complement

In vivo, human immunodeficiency virus type 1 (HIV-1) is opsonized with complement fragments and virus-specific antibodies (Ab). Thus, HIV is able to interact with complement receptor (CR) - and Fc receptor (FcR) - positive cells such as B cells, follicular dendritic cells or macrophages. In this study we demonstrate that the interaction between B cells and HIV has an impact on autologous primary T cell infection in vitro. We confirmed the presence of complement-fragments and virus-specific Ab on serum-treated HIV using a virus-capture assay. In experiments with CR2-specific Ab we showed that the virus/B cell interaction was mainly dependent on CR2. In infection experiments immobilisation of HIV on stimulated tonsil B cells greatly enhanced the infection of interleukin (IL)-2-activated autologous tonsil T cells. Surprisingly, enhancement of T cell infection by B cell-HIV complexes was observed even in the absence of mitogenic stimuli such as PMA and was independent of the addition of exogenous IL-2. Taken together, these results indicate that primary B cells are able to efficiently transmit opsonised HIV to autologous primary T cells and induce a massive enhancement of infection. These in vitro experiments mimic the in vivo situation in the lymphoid tissue and suggest an alternative mechanism for the infection of primary T cells.

Detachment of human immunodeficiency virus type 1 from germinal centers by blocking complement receptor type 2

After the transition from the acute to the chronic phase of human immunodeficiency virus (HIV) infection, complement mediates long-term storage of virions in germinal centers (GC) of lymphoid tissue. The contribution of particular complement receptors (CRs) to virus trapping in GC was studied on tonsillar specimens from HIV-infected individuals. CR2 (CD21) was identified as the main binding site for HIV in GC. Monoclonal antibodies (MAb) blocking the CR2-C3d interaction were shown to detach 62 to 77% of HIV type 1 from tonsillar cells of an individual in the presymptomatic stage. Although they did so at a lower efficiency, these antibodies were able to remove HIV from tonsillar cells of patients under highly active antiretroviral therapy, suggesting that the C3d-CR2 interaction remains a primary entrapment mechanism in treated patients as well. In contrast, removal of HIV was not observed with MAb blocking CR1 or CR3. Thus, targeting CR2 may facilitate new approaches toward a reduction of residual virus in GC.

CD4-Negative cells bind human immunodeficiency virus type 1 and efficiently transfer virus to T cells

The ability of human immunodeficiency virus strain MN (HIV(MN)), a T-cell line-adapted strain of HIV, and X4 and R5 primary isolates to bind to various cell types was investigated. In general, HIV(MN) bound to cells at higher levels than did the primary isolates. Virus bound to both CD4-positive (CD4(+)) and CD4-negative (CD4(-)) cells, including neutrophils, Raji cells, tonsil mononuclear cells, erythrocytes, platelets, and peripheral blood mononuclear cells (PBMC), although virus bound at significantly higher levels to PBMC. However, there was no difference in the amount of HIV that bound to CD4-enriched or CD4-depleted PBMC. Virus bound to CD4(-) cells was up to 17 times more infectious for T cells in cocultures than was the same amount of cell-free virus. Virus bound to nucleated cells was significantly more infectious than virus bound to erythrocytes or platelets. The enhanced infection of T cells by virus bound to CD4(-) cells was not due to stimulatory signals provided by CD4(-) cells or infection of CD4(-) cells. However, anti-CD18 antibody substantially reduced the enhanced virus replication in T cells, suggesting that virus that bound to the surface of CD4(-) cells is efficiently passed to CD4(+) T cells during cell-cell adhesion. These studies show that HIV binds at relatively high levels to CD4(-) cells and, once bound, is highly infectious for T cells. This suggests that virus binding to the surface of CD4(-) cells is an important route for infection of T cells in vivo.